Destabilization of low-frequency modes (LFMs) driven by a thermal pressure gradient in EAST plasmas with q(min) <= 2

Mode structures and excitation conditions for the low-frequency modes (LFMs) have been investigated in experimental advanced superconducting tokamak (EAST) plasmas with q(min) <= 2. Two different stages/categories of the LFM instabilities are observed during the oscillation of annular/central collapse events: (I) the upward sweeping frequency of LFMs; (II) the upward frequency jumpsof LFMs. The annular/central events are triggered by the m/n = 2/1 double tearing modes with different q-profiles, while the LFMs are characterized by higher mode numbers m/n = 4/2, 6/3, ..., where m and n are the poloidal and toroidal mode numbers, respectively. The maximum radial coverage of the LFMs is located in the annular region of 1.97 <= R <= 2.07 m with the normalized minor radius 0.2 <= rho <= 0.4, while the higher-frequency (or upward sweeping frequency) branch is more localized to the radial position of 2 <= R <= 2.02 m (q(min)). The frequency characteristics of upward sweeps or upward jumps of the LFMs are mainly attributed to the change in the q-profile, e.g. the upward sweeping frequency in stage I is caused by q(min) decreasing. Accordingly, the linear wave properties of LFMs in EAST with weak/reversed magnetic shear are studied numerically and analytically based on a general fishbone-like dispersion relation. Without considering the contribution of energetic ions, it is shown that the LFM with Alfvenic polarization is an MHD-unstable kinetic ballooning mode with frequency of the order of the ion diamagnetic drift frequency. Several important factors for the excitation of LFM instability are analyzed: (1) the role of energetic ions is unimportant, and the LFMs can be excited under the two conditions of with/without energetic ions; (2) the higher tau = T-e/T-i with larger eta(i) = L-ni/L-Ti are required, namely the normalized pressure gradient alpha proportional to (1 + tau)(1 + eta(i)) should be large enough to overcome the stability effect of finite field line bending; (3) the weak/reversed shear q-profile with q(min) <= 2 and suitable S (r/q)(q '')(1/2) are required.